During processing, liquid metals, and in particular liquid steel, flow from one vessel, such as a tundish, into another vessel, such as a mold, under the influence of gravity. A nozzle may guide and contain the flowing stream of liquid metal during passage from one vessel to another.
Controlling the rate of flow of the liquid metal during processing is essential. To this end, a regulator or flow controller allowing adjustment of the rate of liquid metal flow is used. A common regulator is a stopper rod, although any type of flow regulator known to those skilled in the art can be used. Thus, a typical continuous steel casting process allows liquid metal to flow from a tundish into a mold, through a nozzle employing a stopper rod for flow regulation.
Referring to FIG. 1, in such a typical continuous steel casting process, a tundish 15 is positioned directly above a mold 20 with a nozzle 25 connected to the tundish 15. A nozzle 25 provides a conduit through which liquid metal 10 flows from the tundish 15 to the mold 20. A stopper rod 30 in the tundish 15 controls the rate of flow through the nozzle 25.
FIG. 2 is a partial schematic view, drawn to an enlarged scale, of an entry portion and a lower portion 4035 of a nozzle bore 45 of the nozzle 25 of FIG. 1. In FIG. 2, the entry portion 35 extends between points 1 and 2. The lower portion 40 extends between points 2 and 3. The entry portion 35 of the nozzle bore 45 is in fluid communication with liquid metal 10 contained in the tundish 15. The lower portion 40 of the nozzle bore 45 is partially submerged in liquid metal 10 in the mold 20.
Returning back to FIG. 1, to regulate the liquid metal flow rate from the tundish 15 into the mold 20, the stopper rod 30 is raised or lowered. For example, the flow of liquid metal 10 is stopped if the stopper rod 30 is lowered fully so that a nose 50 of the stopper rod 30 blocks the entry portion 35 of the nozzle bore 45. As the stopper rod 30 is raised above the fully lowered position, liquid metal can flow through the nozzle 25. The rate of flow through the nozzle 25 is controlled by adjustment of the position of the stopper rod 30. As the stopper rod 30 is raised, the nose 50 of the stopper rod 30 is moved farther from the entry portion 35 of the nozzle bore 45, which increases the open area between the stopper nose 50 and the nozzle 25 allowing a greater rate of flow.
FIG. 3 shows another liquid metal flow system from the tundish 15 to the mold 20. This system has a control zone 55 located between the nose 50 of the stopper rod 30 and the entry portion 35 of the nozzle bore 45. The control zone 55 is the narrowest part of the open channel between the stopper nose 50 and the entry portion 35 of the nozzle bore 45. Liquid metal 10 in the tundish 15 has a static pressure caused by gravity. If the stopper rod 30 does not block the entry of liquid metal 10 into the bore 45 of the nozzle, the pressure of liquid metal 10 in the tundish 15 forces liquid metal 10 to flow out of tundish 15 and into nozzle 25.
When the flow is less than the maximum, the characteristics of the open area of control zone 55 are primary factors in the regulation of the rate of flow into the nozzle 25 and subsequently into the mold 20.
FIG. 4 graphically shows changes in the pressure of liquid metal 10 flowing out of the tundish 15 through the control zone 55 and into the nozzle 25. As shown in FIG. 3, point 60 represents a general location within the liquid metal 10 contained in the tundish 15 upstream of the control zone 55. Point 65 represents a general location within the open bore 45 of the nozzle 25 downstream of the control zone 55. As shown in FIG. 4, the general trend in the pressure of liquid metal 10 between points 60 and 65 is a sharp drop in pressure across the control zone 55. The pressure at 60 is generally higher than atmospheric pressure. The pressure at 65 is generally less than atmospheric pressure, resulting in a partial vacuum.
FIG. 5 illustrates a two-component nozzle, including an entry insert 70 and a main body 75. The entry portion 35 of bore 45 extends from points 21 to 22 to 23, and the lower portion 40 extends from points 23 to 24.
FIG. 6 illustrates a liquid metal flow system, from tundish 15 to mold 20 and incorporates the nozzle of FIG. 5. FIG. 7 illustrates the pressure trend from point 60 to point 65 in the system of FIG. 6. The pressure trend for the system of FIG. 6 basically is the same as that for FIG. 3, including a sharp drop in pressure across control zone 55.
In summary, the nozzles of FIGS. 1, 3 and 6 cause a sharp pressure drop across the respective control zones. This sharp pressure drop causes the flow regulation system to be overly sensitive. An overly sensitive flow regulation system tends to cause an operator to continually hunt, or move the regulator to achieve the correct position so as to adjust the size and/or geometry of the control zone for flow stabilization at a desired rate. Hunting for the proper flow regulation causes turbulence in the entry portion 35 and throughout the bore 45 of the nozzle 25.
Turbulence caused by hunting and also by the partial vacuum/low pressure generated downstream of the control zone accelerate erosion around the control zone. For example, erosion of a nose 50 of a stopper rod 30 and an entry portion 35 of a nozzle bore 45 can occur. The highest rate of erosion generally occurs immediately downstream of the control zone 55. Erosion in and about the control zone 55 exacerbates difficulties associated with liquid metal flow rate regulation. Undesirable changes in the critical geometry of the control zone 55, as a result of erosion, lead to unpredictable flow rate variances, which ultimately can result in the complete failure of a flow regulation system.
Referring again to FIG. 5, for reducing erosion, hence improving flow regulation, in some nozzles the entry insert 70 is generally composed of an erosion-resistant refractory material. However, the addition of the entry insert 70 to the nozzle 40 does not affect the sharp pressure drop across control zone 55, as shown in FIGS. 4 and 7. Thus, flow regulation for conventional nozzles remains overly sensitive to regulator movements, due to the size and shape of the control zone defined thereby, making flow rate stabilization difficult to achieve.
Accordingly, a need exists for a nozzle that minimizes the pressure differential across a nozzle control zone, reducing the corrosive effects thereof and stabilizing the size and shape of the control zone, thereby reducing hunting and increasing flow stability.
The present invention fulfills the above-described need by providing a nozzle with a minimal pressure differential across a nozzle control zone, reducing the corrosive effects thereof and stabilizing the size and shape of the control zone, thereby reducing hunting and increasing flow stability.
To this end, the present invention includes a nozzle for controlling a flow of liquid metal including an entry portion for receiving the liquid metal. A regulator such as a stopper rod is movable from an open position to a closed position with respect to the entry portion for respectively permitting and prohibiting flow through the nozzle. The entry portion and the regulator define a control zone therebetween. A pressure modulator, downstream of the control zone, is adapted to minimize a pressure differential across the control zone. The pressure modulator constricts flow downstream of the control zone.
The invention diminishes the sharp pressure drop across the control zone by modulating the pressure in the nozzle downstream of the control zone, reduces the turbulence of the flow immediately downstream of the control zone, and eliminates over-sensitivity of flow regulation. The nozzle of the present invention can reduce erosion in the region of the control zone and stabilize flow regulation, which improves flow control and mold level control during continuous casting.
Other features and advantages of the present invention will become apparent from the following description of the invention, which refers to the accompanying drawings.